2002ApJ...575..327K

We have carried out 21 cm radio continuum, H76α radio recombination line, and various (¹²CO, ¹³CO, CS, and C³⁴S) molecular line observations of the W31 complex. Our radio continuum data show that W31 is composed of two extended H II regions, G10.2-0.3 and G10.3-0.1, each of which comprises an ultracompact H II region, two or more compact components, and a diffuse envelope. The W31 cloud appears as an incomplete shell on the whole and consists of southern spherical and northern flat components, which are associated with G10.2-0.3 and G10.3-0.1, respectively. For an assumed distance of 6 kpc, the molecular cloud has a size of 48 pc and a mass of 6.2x10⁵ M_☉. The IR luminosity-to-mass ratio and the star formation efficiency are derived to be 9 L_☉/M_☉ and 3%, respectively. These estimates are greater than average values of the inner Galactic plane. We detect two large (16 and 11 pc) and massive (2.1x10⁵ and 8.2x10⁴ M_☉) CS-emitting regions in the northern and southern cloud components. The large amount (48% in mass and 16% in area) of dense gas may suggest that the W31 cloud has the ability to form rich stellar clusters and that star formation has only recently begun. The extended envelopes of both G10.2-0.3 and G10.3-0.1 are likely to be results of the champagne flows, based on the distributions of ionized and molecular gas and the velocity gradient of H76α line emission. According to the champagne model, the dynamical ages of the two H II regions would be (4-12)x10⁵ yr. We find strong evidence of bipolar molecular outflows associated with the two ultracompact H II regions. In the vicinity of the ultracompact and compact H II regions in G10.3-0.1, the ¹²CO J=2-1/J=1-0 intensity ratio is high (1.4), and a small but prominent molecular gas hollow exists. Together, these observations strongly indicate that the H II regions and their ionizing stars are interacting with the molecular cloud. Therefore, it is most likely that recently formed massive stars are actively disrupting their parental molecular cloud in the W31 complex.